Fossil fuel combustion, chlorofluorocarbon relases, and agricultural activities (including deforestation) are the primary antropogenic sources of greenhouse gases. Of the three sources, agriculture is the only one that also has a sink capacity. Thus, an accounting of the net carbon (C) flux is required to properly evaluate agriculture's contribution and to determine the opportunities for emissions mitigation through changes in agricultural practices. Common data sets and a standard accounting method are required to perform country-by-country net C analyses. The research used agricultural census data to determine that U.S. agriculture removed 1.3 Pg of CO2 from the atmosphere, in 1987, in the plants that it produced. The turnover times and the fate of the C were not ascertained. The research also showed that 6.4 Tg of CH4 was emitted from live U.S. agricultural animals. A net C flux was computed, but is incomplete, because rice CH4, plant and animal waste CH4 and CO2, and soil-atmosphere C fluxes could not be estimated from the census data. Additionally, agriculture's net contribution to atmospheric C was found to depend critically on the boundaries of the analysis. (Copyright (c) 1992 Kluwer Academic Publishers.)